'88 1-ton TH400 won't get up to speed

[5Tractorguy]

Want to bounce this off some of you...

Just had the TH400 rebuilt in my '88 for the second time. Stock stall heavy duty converter, heavier kit put in the transmission, nothing too special. All the clutches and plates were changed. Not sure what my truck has for diff gears...

Problem is I can't get up to speed. In other words I'm wound out at 45-50mph turning 3000rpm easy.

A few years ago I used to be able to get up to 65mph at 2800-3000 and head down the highway no problem.

My reason for having the trans rebuilt again was the slow decline in my top speed. I was hoping this rebuild would take care of it but something still doesn't seem right.

As a side thought, would a vacuum leak cause the vacuum modulator (which was replaced) not to shift completely into gear?

Anyone experience this before?
Any thoughts/ideas would be greatly appreciated. Thanks!

[D13]

Can you feel it shift 1-2 then 2-3?
The modulator controls transmission line pressure. That pressure and the governer control the shift points.

Light thorttle I would expect 1-2 around 15mph, 2-3 anywhere from 25-30ish.

If there is no vacuum on the modulator, 1-2 will be late (like 35+Mph) and firm. 2-3 will be 60+. The mosulator is telling the trans your foot is on the floor.

The other thing on a TH400 is the electical down shift solenoid. If it is wired wrong or stuck the trans will always think your foot is on the floor.

Charged my dad a trip to town, a grape nehi soda and $.39 for the vacuum line to fix the high rev late shift oln his TH400. After 10 years the hose had simply rotted off.

[Indyfab]

I had an barely noticeable cut in a 3” section of rubber vacuum line right by the modulator that caused similar symptoms. The hose was new at the time of install, but somehow got sliced .

[5Tractorguy]

Originally Posted by D13

Can you feel it shift 1-2 then 2-3?
The modulator controls transmission line pressure. That pressure and the governer control the shift points.

Light thorttle I would expect 1-2 around 15mph, 2-3 anywhere from 25-30ish.

If there is no vacuum on the modulator, 1-2 will be late (like 35+Mph) and firm. 2-3 will be 60+. The mosulator is telling the trans your foot is on the floor.

The other thing on a TH400 is the electical down shift solenoid. If it is wired wrong or stuck the trans will always think your foot is on the floor.

Charged my dad a trip to town, a grape nehi soda and $.39 for the vacuum line to fix the high rev late shift oln his TH400. After 10 years the hose had simply rotted off.

Yep, that sounds about right for shift points with the MPH, probably a bit more on the early side. 1-2 is very "soft" and 2-3 is more of a hard shift. It will shift through all the gears but doesn't seem to want to get the wheel speed.... if that makes any sense...

I do have the electric down shift in the transmission, but nothing is wired into the truck. I bought the switch a while back but never got time to install it.

Originally Posted by Indyfab

I had an barely noticeable cut in a 3” section of rubber vacuum line right by the modulator that caused similar symptoms. The hose was new at the time of install, but somehow got sliced .

I'll check the hose tomorrow AM before I go out. Has the metal line with the connector hose on ether end. They might be past their prime.

[5Tractorguy]

Checked the line to the vacuum module. Rubber lines on the end weren't bad but got replaced. Metal line is clean and clear. Took fitting for vacuum out of intake manifold, cleaned and resealed threads.

Still drives the same.

I'm beginning to think it's something deep inside the transmission that's messed up...

[GRADYS Performance]

If you can figure out your exact Rpm, Tire size, Rear gear ratio you can figure what your MPH should be. TCIAUTO.com has a calculator to figure rpm, mph etc.
Then you can figure if you have a problem and what component it is, Trans or Converter.

[WorkinLonghorn]

What does the fluid look like? Nice red with no odor? Correct level with engine warm and running in Park?
If you are turning "300RPM easy" at 45-50 MPH it is not about your dif gears or tire size.
It is slipping BADLY. You should NOT attempt to run it in this condition, although probably you have already damaged it severely.
Can you get it towed back to the guys that last worked on it? Did you call them?- Or maybe you did your own work. -BA

[5Tractorguy]

Originally Posted by GRADYS Performance

If you can figure out your exact Rpm, Tire size, Rear gear ratio you can figure what your MPH should be. TCIAUTO.com has a calculator to figure rpm, mph etc.
Then you can figure if you have a problem and what component it is, Trans or Converter.

If it comes to that I'll have to rig up my tach and see if I can figure out what my diff ratio is. Have to jack it up and spin the driveshaft since there is no build sheet on the truck.

Originally Posted by WorkinLonghorn

What does the fluid look like? Nice red with no odor? Correct level with engine warm and running in Park?
If you are turning "300RPM easy" at 45-50 MPH it is not about your dif gears or tire size.
It is slipping BADLY. You should NOT attempt to run it in this condition, although probably you have already damaged it severely.
Can you get it towed back to the guys that last worked on it? Did you call them?- Or maybe you did your own work. -BA

Clean new fluid as of this Thursday. Filled to the correct level when warmed up at idle.

Yep you guessed it, already ran it doing errands and stuff today. Hasn't gotten worse but sure ain't any better.

I pulled the transmission and brought it to someone to rebuild. I'm going to call him tomorrow and see what he thinks. He is a reputable guy via a couple other truck guys in the area that have had transmissions (th400's and 700r4's) done. They haven't had any problems. Who knows, maybe he missed/overlooked something.

I'll have more to report tomorrow...

[sweetk30]

http://www.grimmjeeper.com/gears.html

great calculator and offers side by side .

[5Tractorguy]

Thanks for the link K30!


Got off the phone with the rebuild guy. He's at a loss for what it might be. He is a state away so after the first of the year he's going to take a trip down and take the truck for a ride. We're going go from there. At this point I may pick up another TH400 for a future rebuild.

So until then, that'll be all for now...

[Dead Parrot]

One test you could try is at a stop, put it in L1. Slowly speed up to the point where it should want to shift, then manually shift to L2. Assuming it shifts, continue slowly speeding up to where it should want shift to 3rd. Then manually shift to D. If it shifts but your rpms stay up, then you likely have a slippage problem. If it doesn't shift, then maybe a bad modulator or something inside keeping it from shifting.

[sweetk30]

and a cheep tach to help pin point the rpm to ground speed .

my k30 with 4.10 gears and 235/85/16 stock tires is in 3rd gear by 25mph . . . and at 60mph she sounds like she is screaming @ 2750 rpm

[Alex V.]

FWIW the shift points on my '85 C3500 w/454, TH400, and 4.10 gears is 1-2 @ 6 MPH, 2-3 at 16. That's basically coasting. And yes, the modulator works - highest I've seen my 2-3 shift is about 35 MPH. No electric kickdown.

If it's slipping, when you step on it the engine speed should jump more than a few hundred RPM with no corresponding increase in ground speed and little acceleration.

[5Tractorguy]

Thanks for the ideas guys. I should have some time this week to set up a tach and go for a drive. Been a bit busy around here lately. I'll report back then...